Plasma source for spectrometry

ABSTRACT

A plasma source for a spectrometer for spectrochemical analysis of a sample is characterised by use of the magnetic field component of applied microwave energy for exciting a plasma. The source includes a waveguide cavity ( 10 ) fed with TE 10  mode microwave power. A plasma torch ( 16 ) passes through the cavity ( 10 ) and is axially aligned with a magnetic field maximum ( 18 ) of the applied microwave electromagnetic field. Magnetic field concentration structures such as triangular section metal bars ( 20 ) may be provided. In an alternative embodiment a resonant iris may be provided within a waveguide and the plasma torch positioned relative thereto such that the microwave electromagnetic field at the resonant iris excites the plasma.

TECHNICAL FIELD

[0001] The present invention relates to spectrometry and in particularto a method and apparatus for producing a plasma by microwave power forheating a sample for spectrochemical analysis, for example by opticalemission spectrometry or mass spectrometry.

BACKGROUND

[0002] It is known to excite a plasma to heat a sample for optical ormass spectrometry via an axial electric field (that is, axially of theplasma torch) using frequencies in the microwave region (typically 2455Mhz). Examples of known microwave induced plasma (MIP) spectrometers, asdiscussed in U.S. Pat. No. 4,902,099 by Okamoto et al, employ aBeenakker cavity, which utilises a TM₀₁₀ cavity, or a “Surfatron”. Thesesuffer from the disadvantage that the plasma forms in the form of a ballor cylinder. Sample injected into such a plasma is heated directly bythe microwave energy (principally by electron bombardment). Thisexcitation is very vigorous and leads to the production of undesiredinterferences. Also, direct interaction between the microwave energy anda changing sample load can destabilise the plasma. A better approach isto form the plasma in the form of an annulus or hollow tube with thesample injected into the hollow core. The electrical energy isdissipated in the outer layer which consists of pure support gas, andthe sample is heated from this outer layer via thermal conduction andradiation. This isolates the sample from the electrical energy andresults in more gentle excitation.

[0003] The Okamoto et al patent discloses an MIP spectrometer whichprovides a plasma having improved characteristics. The Okamoto et alspectrometer uses an antenna having multiple parallel slots arrangedaround the circumference of a conducting tube which contains a plasmatorch. The antenna is inside a cavity supplied with microwave power ofTE₀₁ mode.

[0004] The present invention in seeking to provide a relatively simpleand inexpensive method and apparatus for producing a plasma forspectrometry which is in the form generally of a hollow cylinder,provides an alternative to the Okamoto et al arrangement.

SUMMARY OF THE INVENTION

[0005] Accordingly, in a first aspect the invention provides a method ofproducing a plasma for spectrochemical analysis of a sample comprising

[0006] supplying a plasma forming gas to a plasma torch,

[0007] applying microwave power to the plasma torch, and

[0008] relatively positioning the plasma torch to axially align it witha magnetic field maximum of the microwave electromagnetic field, whereinthe applied microwave power is such as to maintain a plasma of theplasma forming gas for heating a sample entrained in a carrier gas forspectrochemical analysis of the sample.

[0009] In a second aspect, the invention provides a plasma source for aspectrometer comprising

[0010] microwave generation means for generating microwave power,

[0011] a waveguide for receiving and supplying the microwave power,

[0012] a plasma torch having passages for supply of respectively atleast a plasma forming gas and a carrier gas with entrained sample,

[0013] wherein the plasma torch is positioned relative to the waveguidesuch that it is substantially axially aligned with a magnetic fieldmaximum of the microwave electromagnetic field for excitation of aplasma of the plasma forming gas for heating the sample forspectrochemical analysis.

[0014] An axial magnetic field induces tangential electric fields whichin turn induce circulating currents in the conducting plasma. Thesecirculating currents induce a magnetic field which opposes the appliedfield and shields the core of the plasma region from the applied field.As a consequence, most of the current flows in the outer layer of theplasma creating the cylindrical shape required. The effect is known andis often referred to as the “skin effect”.

[0015] A considerable field strength is required in order to initiateand sustain the required plasma. This field strength is more readilyachieved with a moderate sized microwave power source by use of aresonant cavity. Such a cavity stores energy at the resonant frequencyand thus raises the peak field strength available for the same level ofsupplied microwave power. The degree to which this occurs is defined bythe quality factor or Q of the cavity and Q's>=10 have proven effective.A particularly preferred requirement of a cavity for this invention isthat it produce a magnetic field maximum in an unencumbered region ofspace so that a plasma torch can be inserted at the magnetic fieldmaximum. Many possible cavities exist and are described in appropriatemicrowave texts, for example “Microwave Engineering” by Peter A RizziISBN 0-13-586702-9 1988 Prentice Hall.

[0016] A simple yet effective approach is to use a cavity formed from alength of waveguide short circuited at one end and fed with microwavepower via a suitable iris from the other end. Such a cavity operates inthe TE_(10n) mode (where n is an integer that depends on cavity length).This also has the advantage of being readily fed with microwave powertransmitted in the TE₁₀ mode which is the most common and simplest wayof transmitting microwave power along a waveguide. Cavities with a low Qoffer the advantage of broad and therefore simple tuning. However theymay not offer enough increase in magnetic field strength for optimummaintenance of the desired plasma. To this end magnetic fieldconcentration structures may be employed within the cavity to furtherincrease the peak magnetic field strength. In the case of a cavityformed by a waveguide which is short-circuited at one end, these can beconveniently provided by conducting bars (eg: metallic bars) placed incontact with each side of the inside wall of the cavity so as to reducethe cavity height in parallel alignment with the plasma torch.Rectangular bars may be used but preferably the height reduction is mademore gradually for example by use of bars with a triangularcross-section with the apexes directed inwardly.

[0017] Alternatively a resonant iris may be provided within thewaveguide and a plasma torch positioned relative to this iris such thatthe microwave electromagnetic field at the resonant iris excites theplasma.

[0018] Preferably the resonant iris is provided by a structure whichdefines an opening to provide the resonant iris by reducing a width anda height of the waveguide. The structure may be a metal section having athickness dimension along the waveguide with the plasma torchaccommodated within a through hole of the metal section which intersectsthe resonant iris opening.

[0019] According to a third aspect, the invention also provides awaveguide for a microwave induced plasma source for spectrochemicalanalysis of a sample,

[0020] wherein the waveguide is dimensioned to operate in the TE₁₀ modeand includes apertures for accommodating a plasma torch, wherein theapertures are located such that in use a plasma torch located in thewaveguide and extending through said apertures will be axially alignedwith a magnetic field maximum of the microwave electromagnetic field.

[0021] For a better understanding of the invention and to show how itmay be carried into effect, embodiments thereof will now be described byway of non-limiting example only, with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a schematic diagram of an embodiment of the invention inwhich a waveguide cavity is shown partially broken away to illustrateother components.

[0023]FIG. 2 illustrates a microwave generator, waveguide and cavitystructure for use in the invention.

[0024]FIG. 3 is another embodiment of the invention.

[0025]FIG. 4 shows portion of a waveguide for supplying microwave powerfor an embodiment of the invention.

[0026]FIG. 5 illustrates a resonant iris for use in an embodiment of theinvention.

[0027]FIG. 6 illustrates an embodiment of the invention employing aresonant iris in a waveguide.

[0028]FIG. 7 illustrates portion of another resonant iris for use in anembodiment of the invention.

[0029]FIG. 8 is a cross-sectional view of a plasma torch within aresonant iris within a waveguide according to an embodiment of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] An embodiment of the invention as illustrated by FIG. 1 comprisesa microwave waveguide which is a rectangular cavity 10 within which ispositioned a plasma torch 16 (which is diagrammatically represented as acylinder).

[0031] The rectangular cavity 10 operates in the TE_(10n) mode. It isshort-circuited at one end 12 and fed with TE₁₀ mode microwave power viaa suitable reactive discontinuity such as an iris or post (not shown)from the other end 14. If the electrical length L of the section ofwaveguide 10 with the iris loading is made n/2 wavelengths long (where nis an integer>=1) it will form a resonant cavity. Electric field maximawill occur every (m/2+¼) wavelengths from the short-circuited end 12(where m is an integer between 0 and n−1) and magnetic field maxima willoccur every m/2 wavelengths from the short circuited end 12. Theshortest cavity length which produces a magnetic field maximum in anunencumbered region is for n=2 ie: L=1 wavelength and the cavity modebecomes TE₁₀₂. In a cavity of this length there is a magnetic fieldmaximum ½ wavelength from the short-circuited end 12. Representativemagnetic field lines are referenced 18 in FIG. 1. By placing a plasmatorch 16 substantially at this location, as shown in FIG. 1, axialmagnetic excitation of a plasma forming gas supplied to the torch can bereadily achieved. The plasma torch 16 is only diagrammaticallyrepresented in FIG. 1 as a cylinder because plasma torch structures forspectrometers are well known. Commonly in plasma torches at least twoconcentric tubes (typically of quartz) are used. A carrier gas withentrained sample normally flows through the innermost tube and aseparate plasma sustaining and torch cooling gas flows in the gapbetween the two tubes. Typically the plasma forming and sustaining gaswill be an inert gas such as argon and arrangements are provided forproducing a flow of this gas conducive to forming a stable plasma havinga hollow core, and to keeping the plasma sufficiently isolated from anypart of the torch so that no part of the torch is overheated. Forexample the flow may be injected radially off axis so that the flowspirals. This latter gas flow sustains the plasma and the sample carriedin the inner gas flow is heated by radiation and conduction from theplasma. An example of a suitable plasma torch is described in detailhereinbelow with reference to FIG. 8.

[0032] Magnetic field concentration structures, namely metal bars 20 areaffixed to and in intimate contact with (with reference to theorientation shown in FIG. 1) the top 22 and bottom 24 inside surfaces ofthe cavity 10 but do not contact the side walls 26 and 28. Thesestructures 20 direct more of the magnetic flux through the regionoccupied by the torch 16. As described hereinbefore, the bars 20 may berectangular in cross section but preferably, the change in cavity heightdue to the bars 20 is made more gradually. This may be achieved bymaking the cross section of the bars triangular, or in the form of thechord of a circle, or any other shape which changes thicknessprogressively across the width of the bar to a maximum at the centre ofthe width.

[0033] The iris at the end 14 may be a capacitive iris (i.e. a thinplate which locally reduces the height of the waveguide), or aninductive iris (i.e. a thin plate which locally reduces the width of thewaveguide or a post spanning the height of the waveguide), or a selfresonant iris (i.e. a plate which locally reduces both the height andthe width of the waveguide). Preferably an inductive iris is used.

[0034] Plasma ignition may be facilitated by seeding the high magneticfield region with some ions. These can be conveniently generated by alocalised breakdown of the plasma forming gas, for example via anelectrical spark passing through the torch 16 in the region of highmagnetic field. This method of plasma ignition is known.

[0035] For a plasma torch having an inner diameter of 11 mm, microwavepower levels in the range of a few hundred watts to around 1 kW readilysustain the plasma discharge in argon or nitrogen. Smaller torches wouldrequire less power. Typical dimensions for an aluminium waveguide 10 are80 mm×40 mm outside dimensions with a 3 mm thickness wall. The openingin the inductive iris end 14 is about 40 mm symmetrically positionedacross the 80 mm dimension. Typical field concentrator bars which aretriangular in cross section are 60 mm wide at the base, 9 mm high at theapex and 70 mm long and the cavity length is approximately 216 mm long.

[0036] Microwave generation means such as a magnetron 30 (see FIG. 2)may feed the microwave power into a feeder waveguide 32, also operatingin the TE₁₀ mode. A resonant cavity 10 (as in FIG. 1) is attached to thefeeder waveguide 32 via respective clamping flanges 34 and 36, betweenwhich a plate 38 providing the preferred inductive iris is clamped.

[0037]FIG. 3 shows an embodiment which is realised using a single lengthof waveguide. In this embodiment a length of rectangular waveguide 40 isshort-circuited at both ends 42, 44 and a magnetron 46 is mounted theappropriate distance from one short-circuited end 44. Two slots areformed in the waveguide 40 one electrical wavelength from the othershort-circuited end 42 and metal plates 48 are welded into these slotsto form the required inductive iris 50. The portion of waveguide 40between end 42 and plates 48 forms a resonant cavity 52. As in the FIG.1 embodiment, a plasma torch 54 (also shown diagrammatically as acylinder) is located substantially half a wavelength from theshort-circuited end of the cavity 52 for excitation of a plasma in aplasma forming gas by the magnetic field of TE₁₀ mode microwave powersupplied by waveguide 40. Magnetic field concentration bars 56 are alsoincluded. Impedance matching stubs 58 may be included in the waveguidesection 40. A tuning stub may be incorporated into cavity 52 ifnecessary, (for example in face 42 (not shown).

[0038] As an alternative to the plates 48 providing an iris 50 as in theFIG. 3 embodiment, a post 60 may be provided as shown in FIG. 4. Post 60is a metal rod which must electrically contact the top wall 62 andbottom wall 64 inner surfaces of the waveguide 40. Provision of a post60 is simpler and cheaper than the plates 48 of FIG. 3 as it involvesmerely drilling a hole through the top and bottom walls 62, 64,inserting the metal rod 60 and either bolting or welding it in position.Example dimensions for a waveguide 40 as in FIG. 4 are interiordimensions 34 mm height×74 mm width, post 60 of 3-4 mm diameter passingalong the 34 mm height and positioned in the middle of the 74 mm widefaces.

[0039] Another embodiment of the invention (see FIGS. 5 and 6) comprisesa waveguide 70 within which is positioned a resonant iris 72 (providedby an opening in a metal section 78) having a plasma torch 74 locatedwithin the iris. The resonant iris 72 is positioned in waveguide 70 suchthat the torch 74 will be substantially axially aligned with a magneticfield maximum of the applied microwave electromagnetic field. Themicrowave power may be supplied to end 76 of waveguide 70 by a microwavegeneration means such as a magnetron (not shown, but similar to amagnetron 30 or 46 as shown in FIGS. 2 and 3 respectively).

[0040] Standard texts on microwave systems describe a number of possiblesections for a resonant iris. A simple and effective example is to use ametal section 78 (see FIG. 5) where the width and height of thewaveguide 70 are simultaneously reduced. The reduced height represents acapacitor and the reduced width represents an inductor. The combinationof a parallel inductor and capacitor forms a resonant circuit. Theapproximate conditions for resonance are that the perimeter of theopening forming iris 72 be an integral number of half-wavelengths long.This is only approximate because the resonant frequency also depends onthe thickness t of the section 78 (i.e. its dimension along thewaveguide 70). In practice the most expedient method of finding theexact size required is to make a trial opening with the perimeter of theopening n half-wavelengths long, where n is an integer, measure theexact resonant frequency and then linearly scale the length l or heighth of the opening to the exact frequency required. Ideally, such anopening should not have sharp corners since these cause undesirablefield and surface current concentrations. A simple solution to this isto make the ends 80 of the opening either radiused or semicircular. Asan example for the 34×74 mm waveguide described hereinbefore, a suitableopening is h=16 mm with semicircular ends 80 (that is, with 8 mm radii),and an overall length of the opening of l=43 mm. Thickness t of thesection 78 is about 18 mm which is enough to accommodate the torch 74.The torch 74 is accommodated in a hole 82 in section 78 such that itpasses through the middle of the iris opening 72 parallel to thedimension l. Hole 82 may be 13 mm in diameter.

[0041] Resonant iris 72 may be located substantially in the middle of awaveguide cavity 70 which is one wavelength long. However it has beenfound that this length of waveguide is not required in that microwavepower may be fed onto iris 72 from one side with the other side openinginto a shorted section of the waveguide 70. Thus the waveguide 70 can beshorted by an end plate 84 (see FIG. 6) which is conveniently locatedsubstantially one half wavelength from the axis of torch 74 (that is,distance x=λ/2). This distance λ/2 places the iris 72 (and thus torch74) substantially at a location where the axial magnetic field is amaximum and the electric field is a minimum. Such a structure causesexcitation of the plasma by both a magnetic field and an electric field(which differs from the embodiments of FIGS. 1-4 where excitation is bythe magnetic field), Such excitation results in a plasma having anelliptical cross section.

[0042] An embodiment using a resonant iris 72 as in FIGS. 5-6 allows fora smaller structure than those of FIGS. 1-4. It also does not requirefield concentration structures such as 20 or 56. Thus a resonant irisbased embodiment such as in FIGS. 5-6 is simpler and cheaper to providethan an embodiment as in FIGS. 1-4.

[0043] The skin depth which defines the region in which electricalenergy is dissipated depends on the degree of conductivity of the plasmaand the microwave frequency. Typically, noble gases such as helium orargon are used to sustain a plasma used for analytical purposes. Boththese gases are easily ionised and as a consequence, the electricalresistivity of the resulting plasma is very low. At 2455 MHz the skindepth of an argon plasma according to the current invention has beenmeasured as about 1 mm. This small depth can result in insufficientheating into the centre region containing the sample unless the torch ismade very small. Use of a gas which exhibits a lower level of ionisationfor the same plasma temperature gives a higher plasma resistivity. Thisin turn gives a greater skin depth improving thermal transfer to thesample-carrying core. Typically a polyatomic gas is suitable. Thepreferred choice is diatomic nitrogen or air due to their low cost andease of procurement, although other gasses may also be suitable. Oneproblem is that the ignition of the plasma is more difficult in diatomicgasses. A solution is to ignite the plasma initially on a monatomic gassuch as argon and switch over to the diatomic gas (for example nitrogen)after the plasma has been created.

[0044] Another practical problem to be addressed in a microwave inducedplasma apparatus according to the invention is that of thermally coolingthe microwave cavity. Whilst this can be done by circulating water orair over the outside of the cavity, a particularly convenient approachis to blow cooling air through the inside of the cavity. Provision of anopening in the end of the cavity allows the hot air to escape and alsoserves as a viewing port to allow a visual check of plasma appearance.Leakage of microwave energy from this opening is avoided by making theopening in the form of a cylindrical tube whose length is at least 2times the diameter. A typical opening may have a diameter of about 20 mmand a tube length of at least 40 mm. Air inlet to the system may be madevia a similar opening in the magnetron launch waveguide.

[0045] A problem with conventional inductively coupled plasma torches isthat the plasma tends to expand to fully fill the confinement tube, thewalls of which could then melt, particularly if made of quartz. Thesolution to this problem is to use a gas sheathing layer to prevent theplasma contacting the walls. For a microwave induced plasma the higherfrequency compared to a conventional radio frequency source of aninductively coupled plasma (ICP) exacerbates this problem. Although gassheathing as in conventional torches may be employed, another solutionis to concentrate the microwave energy in the middle of the torchinstead of substantially uniformly over its full cross-sectional area.This may be achieved by using a modified resonant iris 90 as shown inFIG. 7.

[0046] Iris 90 is provided by an opening in a metal section 92 having areduced height compared to the height h of iris 72 of FIG. 5. The heightof iris 90 is reduced to less than the plasma torch diameter. A hole 94for accommodating the plasma torch passes through the middle of thesection 92. Example dimensions for an iris 90 in section 92 foraccommodating a plasma torch of about 12.5 mm outer diameter are:section 92=74 mm×34 mm×18 mm thickness, iris opening 90=47.7 mm length×8mm height with semicircular ends, hole 94=13 mm diameter.

[0047] A plasma torch for use in the invention may be similar to a known“minitorch” used for ICP applications, except for its outer tube beingextended in length. Thus a torch 100 (illustrated in FIG. 8 asaccommodated within a section 102 providing a resonant iris within awaveguide 103) consists of three concentric tubes 104, 106, 108. Tube104 is the outer tube, tube 106 the intermediate tube and tube 108 theinner tube. Tube 106 includes an end portion of larger diameter toprovide a narrow annular gap between tubes 104 and 106 for the passageof plasma forming gas that is supplied through an inlet 110. The narrowgap imparts a desirably high velocity to the plasma forming gas. Anauxiliary gas flow is supplied to tube 106 through an inlet 112 andserves to keep a plasma 116 formed from the plasma forming gas anappropriate distance away from the nearby ends of tubes 106 and 108 sothat these ends do not overheat. A carrier gas containing entrainedsample aerosol is supplied to inner tube 108 through an inlet 114 and onexiting the outlet of tube 108 forms a channel 118 through plasma 116for the sample aerosol to be vaporised, atomised and spectrochemicallyexcited by the heat of the plasma. As is known, the diameter of innertube 108 is chosen to match the rate of flow of carrier gas andentrained sample aerosol provided by a nebulizer (or other sampleintroduction means) that is used with the torch 100. The velocity of theaerosol laden carrier gas emerging from inner tube 108 must besufficient to make a channel 118 through the plasma 116, but not sogreat that there is insufficient time for the aerosol to be properlyvaporised, atomised and spectrochemically excited. It has been foundthat a nebulizer and spray chamber from a conventional inductivelycoupled argon plasma system performs satisfactorily with the presentinvention when the internal diameter of tube 108 of a torch 100 is inthe range 1.5-2.5 mm.

[0048] Torch 100 may be constructed of fused quartz and have an outerdiameter of approximately 12.5 mm. Its outer tube 104 may be extended inlength to protrude a short distance from the waveguide 103. FIG. 8 showsa torch in which the three tubes 104, 106, 108 are permanently fusedtogether, however a mechanical arrangement may be provided whereby thethree tubes are held in their required positions and wherein one or moreof the tubes can be removed and replaced, as is known. Such anarrangement is called a demountable torch. Torch 100 may be constructedof materials other than quartz, such as for example alumina, boronnitride or other heat resistant ceramics. An embodiment as in FIG. 8readily supports an analytically useful plasma in nitrogen at powerlevels ranging from below about 200 watts to beyond 1 kilowatt.

[0049] The discussion hereinbefore of the background to the inventionand of what is known or conventional is included to explain the contextof the invention and the invention itself. This is not to be taken as anadmission that any of the material referred to was part of the commongeneral knowledge in Australia as at the priority date of the claims ofthis application.

[0050] The invention described herein is susceptible to variations,modifications and/or additions other than those specifically describedand it is to be understood that the invention includes all suchvariations, modifications and/or additions which fall within the scopeof the following claims.

1. A method of producing a plasma for spectrochemical analysis of asample comprising supplying a plasma forming gas to a plasma torch,applying microwave power to the plasma torch, and relatively positioningthe plasma torch to axially align it substantially with a magnetic fieldmaximum of the microwave electromagnetic field, wherein the appliedmicrowave power is such as to maintain a plasma of the plasma forminggas for heating a sample entrained in a carrier gas for spectrochemicalanalysis of the sample.
 2. A method as claimed in claim 1 whereinmicrowave power of TE₁₀ mode is applied to the plasma torch.
 3. A methodas claimed in claim 1 or 2 wherein the plasma is ignited by initiating alocalised break-down of the plasma forming gas within the magnetic fieldregion to produce seeding ions.
 4. A method as claimed in claim 3wherein the localised breakdown is initiated by a spark discharge.
 5. Amethod as claimed in any one of claims 1 to 4 including shaping themagnetic field to increase the magnetic flux concentration which passesaxially of the torch.
 6. A method as claimed in any one of claims 1 to 5wherein the plasma forming gas is a diatomic gas.
 7. A method as claimedin any one of claims 1 to 6 wherein the plasma forming gas is nitrogen.8. A method as claimed in any one of claims 1 to 6 wherein the plasmaforming gas is air.
 9. A method as claimed in claim 6 wherein the plasmais ignited with argon as the plasma forming gas, the diatomic gas beingsubsequently supplied to sustain the plasma.
 10. A method as claimed inany one of claims 1 to 5 wherein the plasma forming gas is argon.
 11. Aplasma source for a spectrometer comprising microwave generation meansfor generating microwave power, a waveguide for receiving and supplyingthe microwave power, a plasma torch having passages for supply ofrespectively at least a plasma gas and a carrier gas with entrainedsample, wherein the plasma torch is positioned relative to the waveguidesuch that it is substantially axially aligned with a magnetic fieldmaximum of the microwave electromagnetic field for excitation of aplasma of the plasma forming gas for heating the sample forspectrochemical analysis.
 12. A plasma source as claimed in claim 11wherein the waveguide is for supplying microwave power in the TE₁₀ mode.13. A plasma source as claimed in claim 11 or 12 wherein the waveguideis a resonant cavity for the supplied microwave power.
 14. A plasmasource as claimed in any one of claims 11 to 13 including fieldconcentration structures within the waveguide for shaping the magneticfield to increase the magnetic flux which passes axially of the torch.15. A plasma source as claimed in claim 14 wherein the fieldconcentration structures are metallic bars aligned parallel with theplasma torch and which span opposite inside walls of the waveguide incontact therewith.
 16. A plasma source as claimed in claim 15 whereinthe metallic bars are triangular in cross section with the apexesdirected inwardly of the waveguide towards the plasma torch.
 17. Aplasma source as claimed in any one of claims 11 to 16 wherein themicrowave power is supplied to the plasma torch via an inductive orcapacitive element contained in the waveguide located between themicrowave generation means and the plasma torch.
 18. A plasma source asclaimed in claim 17 wherein the inductive element is formed by aconductive post which spans opposite surfaces of the waveguide.
 19. Aplasma source as claimed in claim 11 or 12 including a structure withinthe waveguide which provides a resonant iris, wherein the torch islocated relative to this structure such that the microwaveelectromagnetic field at the resonant iris excites a plasma of theplasma forming gas, wherein said structure and thereby said plasma torchare positioned relative to the waveguide such that the torch issubstantially axially aligned with a magnetic field maximum of themicrowave electromagnetic field.
 20. A plasma source as claimed in claim19 wherein said structure is a metal section having a thicknessdimension along the waveguide and which defines an opening across saidthickness dimension to provide said resonant iris by reducing a widthand a height of the waveguide, wherein the opening has a length and aheight and the plasma torch axially spans the length of the opening. 21.A plasma source as claimed in claim 20 wherein the plasma torch isaccommodated within a hole which passes through the metal section andintersects said resonant iris opening.
 22. A plasma source as claimed inclaim 19 or 20 wherein the resonant iris has a height which is less thanthe outer diameter of the plasma torch for concentrating the microwaveenergy substantially towards the central axis of the plasma torch.
 23. Aplasma source as claimed in any one of claims 11 to 22 wherein theplasma torch comprises an outer tube and an intermediate tube providinga passage therebetween for supply of the plasma gas, and an inner tubewithin the intermediate tube for supply of the carrier gas withentrained sample, wherein the outer tube extends in length beyond theintermediate and inner tubes.
 24. A plasma source as claimed in claim 23wherein the outer tube extends to protrude a short distance from thewaveguide.
 25. A waveguide for a microwave induced plasma source forspectrochemical analysis of a sample, wherein the waveguide isdimensioned to operate in the TE₁₀ mode and includes apertures foraccommodating a plasma torch, wherein the apertures are located suchthat in use a plasma torch located in the waveguide and extendingthrough said apertures will be axially aligned with a magnetic fieldmaximum of the microwave electromagnetic field.
 26. A waveguide asclaimed in claim 25 wherein the waveguide includes structures forconcentrating the magnetic field strength at the plasma torch location.27. A waveguide as claimed in claim 26 wherein said structures areoppositely located conducting bars which contact opposite facingsurfaces of the waveguide and reduce the height dimension of thewaveguide in parallel alignment with the axial direction of the plasmatorch location.
 28. A waveguide as claimed in claim 27 wherein theconducting bars have a triangular cross section with the apexes directedinwardly towards each other.
 29. A waveguide as claimed in claim 25wherein the waveguide includes a structure which defines a resonantiris, wherein said structure includes a through hole for accommodating aplasma torch, the through hole being aligned with said apertures.
 30. Awaveguide as claimed in claim 29 wherein said structure defines anopening to provide said resonant iris by reducing a width and a heightof the waveguide, wherein the resonant iris opening intersects saidthrough hole.
 31. A plasma source for a spectrometer including awaveguide containing a resonant iris, a plasma torch associated with theresonant iris such that a microwave electromagnetic field can be appliedto the resonant iris via the waveguide and for a magnetic field maximumof the electromagnetic field in the resonant iris to be substantiallyaxially aligned with the plasma torch for exciting a plasma in a plasmaforming gas that passes through the plasma torch.
 32. A plasma source asclaimed in claim 31 wherein the resonant iris is a metal section thatcontains a through hole, the plasma torch being accommodated in thethrough hole.
 33. A plasma source as claimed in any one of claims 11 to32 wherein the waveguide includes at least one hole in an end thereoffor passage of cooling air through the waveguide and to provide aviewing port for visual inspection of a plasma formed by the plasmatorch.